1. Field of the Invention
The invention relates generally to navigation systems and more specifically to global positioning system (GPS) devices that use dead-reckoning apparatus to fill in as backup during periods of GPS shadowing such as occur amongst obstacles, e.g., tall buildings in large cities.
Global positioning system receivers typically use signals received from three or more overhead satellites to determine navigational data such as position and velocity, and such systems may also provide altitude and time. GPS signals are available worldwide at no cost and can be used to determine the location of a vehicle, such as a car or truck, to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites to generate pseudo range measurements (PR) from precision code (P-code) or coarse acquisition code (C/A-code) modulation on those carriers. Carrier L1 is allocated to 1575.42 MHz and carrier L2 is positioned at 1227.78 MHz. Less expensive receivers tune to only one carrier frequency, and therefore cannot directly observe the local ionospheric delays that contribute to position error. At such frequencies, radio carrier signals travel by line-of-sight, thus buildings, mountains and the horizon can block reception.
The constellation of GPS satellites in orbit about the earth comprises individual satellites that each transmit a unique identifying code in a code multiple access arrangement. This allows the many GPS satellites to all transmit in spread spectrum mode at the same frequency (plus or minus a Doppler shift from that frequency as results from the satellite's relative velocity). Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 "chip" code to a set of predefined codes that are preassigned to individual GPS satellites. These codes do not arrive in phase with one another at the receiver. Therefore, "finding" a GPS satellite initially involves searching various carrier frequencies, to account for Doppler shift and oscillator inaccuracies, and searching for a code match, using 1023 different code phases and up to twenty-four possible correlation code templates.
In large cities with many tall buildings, one or more of the GPS satellites that a particular receiver may be tracking may be temporarily blocked. In some situations, such blockage can prevent all the overhead GPS satellites from being tracked and such outages can last for several minutes. GPS signals also become unavailable to vehicles when moving through underground or underwater tunnels. Therefore a method and apparatus are needed to bridge an information gap that exists between periods of GPS signal availability. Dead-reckoning techniques have been used in the background art to supply navigation data, both alone and in concert with GPS systems.
The prior art has not recognized nor taken full advantage of the fact that while within the typical "urban canyon," at least two GPS satellites are typically visible at any one instant. A significant performance advantage is possible if such GPS satellites are productively used to blend partial GPS information with dead-reckoning information. Such blending reduces the drift that is inherent in dead-reckoning. More accurate information is thus available on average, and overall accuracy can be maintained for relatively longer periods of GPS signal shadowing.
A navigation system for a vehicle using a dead-reckoning system can encounter several sources of error. Initial position errors can result from GPS inaccuracies, especially in selective availability (SA) and multipath signal environments. A heading error may result from a difference between a vehicle's change in direction and the sensed change in direction, for example, as derived from a single-degree of freedom inertial gyro. Such errors can range from one to five percent for low-cost gyros. Heading errors can also stem from gyro rate bias/drift, scale factor non-linearity and initial warm-up problems. An odometer error is created by differences between the distance a vehicle actually travels and the vehicle's odometer indicated distance. Such errors can be classified as scale factor and scale factor non-linearity. Sensor measurement noise will also corrupt data obtained. Terrain sloping can cause a third type of error in that the ground traveled by a vehicle may exceed the horizontal distance traversed due to a change in altitude.